Subthalamic nucleus deep brain stimulation driven by primary motor cortex γ2 activity in parkinsonian monkeys

In parkinsonism, subthalamic nucleus (STN) electrical deep brain stimulation (DBS) improves symptoms, but may be associated with side effects. Adaptive DBS (aDBS), which enables modulation of stimulation, may limit side effects, but limited information is available about clinical effectiveness and efficaciousness. We developed a brain-machine interface for aDBS, which enables modulation of stimulation parameters of STN-DBS in response to γ2 band activity (80-200 Hz) of local field potentials (LFPs) recorded from the primary motor cortex (M1), and tested its effectiveness in parkinsonian monkeys. We trained two monkeys to perform an upper limb reaching task and rendered them parkinsonian with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine. Bipolar intracortical recording electrodes were implanted in the M1, and a recording chamber was attached to access the STN. In aDBS, the M1 LFPs were recorded, filtered into the γ2 band, and discretized into logic pulses by a window discriminator, and the pulses were used to modulate the interval and amplitude of DBS pulses. In constant DBS (cDBS), constant stimulus intervals and amplitudes were used. Reaction and movement times during the task were measured and compared between aDBS and cDBS. The M1-γ2 activities were increased before and during movements in parkinsonian monkeys and these activities modulated the aDBS pulse interval, amplitude, and dispersion. With aDBS and cDBS, reaction and movement times were significantly decreased in comparison to DBS-OFF. The electric charge delivered was lower with aDBS than cDBS. M1-γ2 aDBS in parkinsonian monkeys resulted in clinical benefits that did not exceed those from cDBS. However, M1-γ2 aDBS achieved this magnitude of benefit for only two thirds of the charge delivered by cDBS. In conclusion, M1-γ2 aDBS is an effective therapeutic approach which requires a lower electrical charge delivery than cDBS for comparable clinical benefits.

www.nature.com/scientificreports/ implementation of the cortically generated plans (Fig. 1B) 30 . During STN-DBS in PD condition, the threshold is decreased, which increases the likelihood for a plan to be selected (Fig. 1C,D,E) 31,32 . In this view, cDBS may decrease by an offset the threshold of the GPi function (Fig. 1C), and STN-β aDBS may decrease the threshold of the GPi function depending of the hypokinetic state fluctuations in time (Fig. 1D). However, it can be argued that nominal competition between plans presupposes a temporal organization between the generation of the plans in the cortex and the modulation of the threshold of the GPi function (Fig. 1A). For this reason and in PD condition, M1-γ aDBS may lower the threshold of the GPi function when plans are presupposed to be generated in the M1 (Fig. 1E) and, therefore, improves the activation function of the basal ganglia.
In the present study, we developed a brain-machine interface to positively modulate the rate and amplitude of the stimulation train of STN-DBS as function of γ2 activity captured in the local field potentials (LFPs) of the M1. The M1-γ2 aDBS was tested in two PD monkeys during a simple motor task. This study was aimed to establish a proof of feasibility and clinical benefits. A comparison to cDBS is provided to highlight the dynamical aspect of M1-γ2 aDBS, although this pre-clinical study was not designed for a robust comparison of efficacy between aDBS and cDBS. Our collective data show that M1-γ2 aDBS is clinically beneficial in PD condition.

Materials and methods
Animals. The experimental protocol was approved by the Institutional Animal Care and Use Committees of the National Institutes of Natural Sciences (Okazaki, Japan) and in accordance with ARRIVE guidelines. All methods were carried out in accordance with relevant guidelines and regulations. We used 2 female Japanese monkeys (Macaca fuscata; monkey A, 5.0 kg; monkey B, 5.4 kg). Body weight and daily living activities were monitored throughout the study.
Behavioral task and training. A simple vertical hand reaching task was used. The monkeys sat facing a plastic tower secured in front of the monkey's chair ( Fig. 2A). A green light emitting diode (LED) was embedded as a visual target in the tower at eye level. A horizontal platform was secured as a home key at the base of the tower, 15 cm below the LED, and enabled the monkey to rest her hand comfortably with the elbow flexed at 90 degrees. Home key and target LED were equipped with infrared optoelectrical sensors (Keyence, Osaka, Japan) to monitor hand movement.
The task was started after the subject rested her hand on the home key for at least 1 s (Fig. 2B). When cued by the visual and auditory signals (0.3 s), the monkey was required to reach and touch the target with her hand within 6 s. Successful completion of this task was rewarded by a sip of water 0.1 s after touching the target. The monkey was allowed 12 s to return her hand to the home key. When the monkey returned her hand to the area near the home key, the experimenter positioned her hand on the home key to initiate the next trial. We defined 4 periods in the task: (1) Rest was 1 s period before the visual and auditory stimuli; (2) Premovement began when the visual and auditory stimuli were triggered and ended when her hand moved off the home key; (3) Movement began when the monkey moved her hand off the home key and ended when she touched the target; and (4) Return began when the monkey removed her hand from the target and ended when she returned her hand to or near the home key (Fig. 2B).
Monkeys were trained 5 days a week for 4 months prior MPTP treatment. We defined a series as 25 trials with successful reach to target, with or without returning to home key. Subjects were deprived of water for 24 h before sessions, and water intake was adjusted with fresh fruits and subcutaneous fluid as needed.
Surgery for head fixation and electrode implantation. Under general anesthesia with ketamine hydrochloride (5-8 mg/kg body weight, i.m.), xylazine hydrochloride (0.5-1 mg/kg, i.m.), and propofol (6-7 µg/ ml, blood concentration, i.v.), surgery was performed to affix pipes to the skull for securing the head to a stereotaxic frame during experiments 33,34 . After 10 days, the cortical recording electrodes were implanted. After securing the head in the stereotaxic frame under general anesthesia with ketamine hydrochloride (5-8 mg/kg body weight, intramuscular) and xylazine hydrochloride (0.5-1 mg/kg, intramuscular), the skull was removed over the M1 contralateral to the hand used for task performance. The forelimb regions of M1 were identified with electrophysiological mapping 33,34 . The cortical recording electrodes were made of 200 µm-diameter Tefloncoated stainless-steel wire (California Fine Wire Co, USA). The wires were paired with matching impedance (≤ 1 k Ohm) for optimal common mode rejection 35 . They were implanted with an inter-electrode distance of 2 mm into layers III-V of the M1 forelimb region for later LFP recording and stabilized mechanically with acrylic resin. A rectangular plastic chamber that covered M1 was fixed to the skull with acrylic resin.
Induction of parkinsonism. The monkeys were treated with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP; Sigma, St Louis, MO, USA) to induce moderate to severe parkinsonism with asymmetric severity. After induction of general anesthesia with ketamine hydrochloride (5-8 mg/kg body weight, i.m.), xylazine hydrochloride (0.5-1 mg/kg, i.m.), and propofol (6-7 µg/ml, blood concentration, i.v.), the external, internal, and common carotid arteries, contralateral to the hand used for task performance, were dissected at the neck region. The external carotid artery was clamped temporarily, and MPTP dissolved in saline (2 mg/mL) was injected into the common carotid artery (monkey A, 0.6 mg/kg; monkey B, 1.3 mg/kg). Later, they received additional intramuscular MPTP injections (monkey A, 0.6 mg/kg/injection, 5 injections) or additional intravenous MPTP injections (monkey B, 0.8 and 0.7 mg/kg, 2 injections) thorough the saphenous vein under general anesthesia with ketamine hydrochloride (5-8 mg/kg body weight, i.m.) and xylazine hydrochloride (0.5-1 mg/kg, i.m.). The total dose of MPTP received by monkey A was 3.6 mg/kg and monkey B was 2.8 mg/kg. Electrophysiological recordings in parkinsonian state were started at 5-8 weeks after the last treatment, when parkinsonian symptoms had stabilized for 2 weeks. The severity of parkinsonism was scored on the side contralateral to the Cortical electrophysiological recording and conversion to stimulation pulses. Behavioral task and electrophysiological recordings were performed in drug-off state (at least 12 hours after withdrawal).The monkey was seated in the monkey chair with the head restrained in a soundproof room shielded with copper nets to block external electromagnetic noise. The head amplifier (Cerebus, Blackrock Microsystems, Salt Lake City, UT) was placed 10 cm from the recording chamber and powered by direct current (12 V) amplified LFPs www.nature.com/scientificreports/ from cortical recording electrodes in the M1. From there, the signal was directed toward two distinct circuits (Fig. 3A). The first circuit consisted of the brain-machine interface aimed to modulate STN-DBS parameters as a function of γ2 activity from the M1 through the online processing system. The second circuit consisted of the data recording setup aimed to digitize and store data from the head stage, the brain-machine interface, the stimulator, and behavioral device for the off-line analyses of the experimental results 26 through the offline processing system. For the brain-machine interface, the M1-LFPs were amplified (x 10K, 50-300 Hz, MEG-6116/AB610J, Nihon Kohden, Tokyo, Japan) and filtered (FV-664, NF Corporation, Yokohama, Japan) to extract γ2 activity (80-200 Hz) (Fig. 3A). After preparing the primate for a recording session and prior to starting the first trial series, the M1-LFPs were recorded for 15 min while the monkey was free to move her arms, legs and trunk. This allowed the experimenter to determine the maximum oscillation amplitude in the γ2 band. A window discriminator  www.nature.com/scientificreports/ (EN-611 J, Nihon Kohden) detected γ2 oscillations with amplitude > the 75th percentile of the maximum γ2 oscillation amplitude and duration > 0.5 ms. The transistor-transistor logic (TTL) pulses generated by the window discriminator as a function of the γ2 band activity were routed to a computer through a digital input and analog output card (PCI6713, National Instruments, Austin, TX) for further processing. The instantaneous frequency of the γ2-driven pulses was averaged over a 50 ms window to modulate the frequency (f) and amplitude (A) of the stimulation train according to (Fig. 3A): and where μ(TTL pulse) was the average rate of TTL pulses on a 50 ms window, Threshold f was the threshold for frequency (set at 50 Hz), Threshold A was the threshold for amplitude (set at 0.50 mA), Gain f was the gain of frequency, and Gain A was the gain of amplitude. The Gain f and Gain A were determined to maintain frequency and amplitude less than the defined high limits. Pulses with frequency f, amplitude A, and pulse width 60 µs were generated by the computer through a digital input and analog output card (National Instrument, PCI6713) and delivered to the DBS lead through an analog isolator (BSI-950, Dagan, Minneapolis, MN). The pulse width was set for both aDBS and cDBS at 60 µs because it did not result in artifact on M1-LFPs and is used in clinical application. We set the frequency range between 50 and 150 Hz because there is controversy about the clinical benefit associated with low-frequency stimulation [37][38][39][40] . Preliminary experiments showed that constant stimulation with amplitude 1 mA resulted in optimal performance. When stimulation exceeded 2 mA, performance decreased, and above 3 mA muscle pulling and dyskinesia were observable. These side effects were interpreted as electrical current spread toward adjacent areas of the STN such as the internal capsule. Therefore, the stimulation amplitude was set at 1 mA for cDBS and ranged from 0.5 to 1.5 mA for aDBS. We compared results with aDBS, cDBS (frequency, 100 Hz; amplitude, 1 mA; pulse width, 60 µs), and DBS-OFF (without DBS).
Stimulation of the STN. The DBS electrodes were tungsten wire electrodes coated by epoxy resins (Fig. 3B; OM210-033a, Unique Medical, Tokyo, Japan) with a distal tip composed of a high impedance electrode (~ 500 kΩ) suitable for multiunit recordings. Additionally, these electrodes included 3 proximal contacts (inter-tips distance, 0.75 mm) with low impedance (~ 10 kΩ) suitable for stimulation. The DBS electrode was inserted either vertically or obliquely (tilted anteriorly by 36 degrees from the vertical in the sagittal plane) to the STN along the same trajectory in each experimental session. The dorso-lateral motor subregion of the STN was identified by sensory responses to passive joint movements of the arm using multiunit recording from the high impedance tip contact. The two proximal contacts were placed in the dorso-lateral motor subregion of the STN and used for bipolar stimulation.
Postprocessing of LFPs recorded from the M1. For the off-line analyses, the M1-LFPs were recorded as bipolar signals (anterior minus posterior LFP), filtered between 3 and 200 Hz using a double reverse Butterworth filter (− 3 dB frequency of 200 Hz with 0 phase shift), stored on the computer, and down-sampled to 500 Hz. Signals were inspected visually and those with more than 3% artifacts in duration were discarded. The M1-LFPs were analyzed first with DBS-OFF condition to define the dynamics of the β, γ1, and γ2 frequency bands. We postulated that change in oscillatory activity related to the task should be band specific, of small amplitude and with high dynamics. We used a dynamic autoregressive model based on a Kalman smoother for tracking the instantaneous frequency in the β (13-30 Hz), γ1 (30-80 Hz) and γ2 (80-200 Hz) frequency bands of the M1 LFPs 26,41,42 .
The instantaneous frequencies were investigated during the 4 task periods which were aligned and normalized in time (100 points per period). The concatenation of the 100 points/period enabled synchronized trials in time for each period (Fig. 4). Graphical representations of the instantaneous frequencies and statistical analyses were performed on the synchronized trials. In PD state, the M1-LFPs were analyzed by their power in the β, γ1, and γ2 frequency bands 26 for comparison to the method based on Kalman filter. Power spectrum density was calculated over the time defined by each task period for each trial. The power spectrum for each period was normalized to the Rest period. www.nature.com/scientificreports/ Statistical analyses. All data were reported as median and percentile range (25th-75th percentiles).
Comparisons between data from different conditions were performed using the nonparametric Kruskal-Wallis and Mann-Whitney tests for pairwise comparisons with P < 0.05 43 adjusted by the Benjamini-Hochberg correction 44 . Page trend test 45 was applied to the time series of instantaneous frequencies to test the null hypothesis that the ordered series was monotonic (aka. no trend) vs. the alternative hypothesis that the series increased with P = 0.95.

Results
Severity of parkinsonian symptoms and task performance. After MPTP treatment, the two monkeys had cardinal signs for parkinsonism including akinesia, bradykinesia, rigidity, and flexed posture, more severe on the side contralateral to the carotid artery MPTP injection. The primate parkinsonian rating scale scores were stable throughout the experimental period (

M1 LFPs during task in DBS-OFF. First, we investigated which band activity can be used as a biomarker
for movements among the β, γ1, and γ2 frequency bands in the M1. Instantaneous frequency analysis of M1-LFPs in DBS-OFF showed that β, γ1, and γ2 powers were altered along the task periods (Fig. 4). There was a trend for β activity to decrease during the Premovement and Movement periods (Page's test, P > 0.99) but not during the Return period (Page's test, P < 0.01) (Fig. 4A). The γ1 did not show a robust monotonic trend during the Premovement period (Page's test, P < 0.01) but there was a downward trend during the Movement period (Page's test, P > 0.99) and no trend during the Return period (Page's test, P < 0.55) (Fig. 4B). The γ2 exhibited a Q = intensity × pulse width × frequency   (Fig. 4C). Power spectrum density during each task period (Fig. 5A) showed that median β power was reduced during the Premovement, Movement, and Return periods (P < 0.001). The median γ1 power for both monkeys combined was unchanged during the Premovement period but increased during the Movement and Return periods, however the median γ1 power in monkey B was unchanged during the Movement period (P > 0.5). The median γ2 power was increased during the Premovement, Movement, and Return periods in each individual monkey (P < 0.0002) (Fig. 5B&C). These changes in the dynamics and median power of γ2 suggest that M1-γ2 activity is a suitable biomarker for movements in parkinsonian condition. With the exception of γ1 activity, Kalman filter and FFT analyses were in agreement.
The dispersions for the pulse intervals (Fig. 6Ac) and amplitudes (Fig. 6Ad) were modulated during the task. The dispersion of the pulse interval decreased during the Premovement (− 29.41%, P < 0.001) and Movement (− 64.71%, P < 0.001) periods, and increased during the Return period (50.01%, P < 0.001) remaining above that during the Rest period (29.41%, P < 0.001, Fig. 6Ac). The dispersion of the stimulus amplitude decreased during the Premovement (by 13%, P < 0.001) and Movement (by 38%, P < 0.001) periods. It increased during the Return period (by 33%, P < 0.001) and remained 10% below its value at the Rest period (P < 0.001, Fig. 6Ad). Changes in the central tendency and dispersion of stimulus intervals and amplitudes were similar between the two monkeys (Figs. 6B&C).
The M1 γ2 activity was increased during the Movement period and decreased during the Return period with aDBS and cDBS (P < 0.001) (Fig. 7A) in both monkey A (Fig. 7B) and monkey B (Fig. 7C).

Behavioral changes by a DBS and cDBS. Success rates to reach the target were similar in both aDBS
and cDBS (91.5%, Table 2) and success rate to return to the home key was higher with cDBS (75.9% vs. 80.7%). Monkey B was mostly responsible for this finding. The duration of each task period was compared between aDBS and cDBS (Table 3). With aDBS, the Premovement and Movement durations were reduced (− 20.1% and 15.8% P < 0.001), but the Return duration was unchanged (P > 0.05). With cDBS, the Premovement and Movement durations were reduced (− 12.0% and − 10.0%, P < 0.001), and the Return duration was increased (+ 6.5%, P < 0.001). aDBS and cDBS were clinically beneficial during the Premovement and Movement periods. aDBS, in comparison to cDBS, was found to be slightly more efficient during the Premovement (8.0% difference, P < 0.001) and Movement (5.8% difference, P < 0.01) periods. Differences in clinical effects between aDBS and cDBS appear during the Return period, with an increase duration in monkey B (by 33.2%, P < 0.001) but not in monkey A (P > 0.05).

Discussion
The results confirmed the feasibility of a brain-machine interface in controlling the stimulus intervals and amplitudes of STN-DBS driven by the fast M1-γ2 oscillatory activity (Fig. 1E) in parkinsonian monkeys during a motor task. The M1-γ2 activity was robustly increased during the planning and execution of movement and successfully modulated the stimulus intervals and amplitudes of STN-DBS during task execution. The stimulus interval was minimized, the stimulus amplitude was maximized, and the dispersions of stimulus intervals and amplitudes were minimized during the task. Both aDBS and cDBS improved task performance over DBS-OFF, with aDBS delivering a lower electrical charge than cDBS.
There are several limitations of the present experimental paradigm that need to be acknowledged to adequately frame the discussion of our findings. One is the lack of feedback on side effects from DBS in an animal model. By comparison and in patients, cDBS is individually adjusted to optimize the benefits-to-side effects ratio because of the variations in anatomic structures and the extent of the electrical current spreading to adjacent structures 46 . The resulting side effects of the electrical current spread include sensorimotor, cognitive, and psychiatric effects [47][48][49] which are generally subtle and verbally reported by the patients; such subtle events may be missed in studies with monkeys. In the current primate study, we carefully minimized the possibility for the occurrence of such side effects by using stereotactic guidance, electrophysiological mapping, and sensorimotor testing in each session to confirm the location of DBS lead contacts in the dorsolateral motor region of the STN. Also, our preliminary observations showed that cDBS at 150 Hz and over 2 mA occasionally induced side effects; therefore, we limited the frequency and amplitude for cDBS to 100 Hz and 1 mA, and those for aDBS to 50-150 Hz and 0.5-1.5 mA. A second limitation is the pulse width of 60 µs in both aDBS and cDBS. This duration of pulse width is commonly used in the clinical application of DBS; importantly, this short pulse width caused minor artifacts in M1-LFPs. This is likely due to the combination of optimizing the recording electrodes for common mode rejection, the short distance between paired electrodes, and the filters and sampling rate www.nature.com/scientificreports/ for LFPs recording. Longer pulse widths (> 200 µs) will likely require additional processing for the rejection of stimulus artifacts, which might cause reverberating effects in the close-loop system. For the reasons cited above, defining the optimal settings for cDBS (or aDBS) may be an elusive task in primate models. Therefore, we tested the two monkeys with the same stimulus parameters for the sake of the comparison between conditions and performed statistical analyses. To be sure, the present study may not be considered as a comparison of optimal clinical benefits between M1-γ2 aDBS and cDBS. Rather, this study supports the feasibility and compares the charge delivery between M1-γ2 aDBS and cDBS. Our data justifies further preliminary clinical studies during DBS surgical procedures in order to more formally compare between M1-γ2 aDBS and cDBS. In fact, M1-γ2 activity successfully modulated the stimulus parameters of STN-DBS and improved PD motor function over DBS-OFF, with improved reaction and movement times, consistent with clinical benefits against bradykinesia and rigidity 50 . Elucidating the physiological mechanisms underlying the benefits from M1-γ2 aDBS is beyond the scope of this study. Nevertheless, our clinical observations may help to narrow the scope of future investigations aimed towards elucidating its mechanisms. One can start by looking at the kinetic profile of DBS therapy. Clinical benefits from DBS are known to show hysteresis 51 . When DBS is turned on or off in PD patients, changes in tremor, rigidity and bradykinesia are delayed by several seconds or minutes 52,53 despite immediate changes in basal ganglia circuitry dynamics 51 . To be sure, we cannot exclude that these lag times may be decreased when DBS patterns are modulated rather than switched between ON and OFF states. Therefore, clinical benefits from aDBS may result from 'near immediate' effects from the stimulation when applied during movement. Alternatively, but not exclusively, clinical benefits from M1-γ2 aDBS during voluntary movement may result from delayed effects from stimulation delivered prior to the voluntary movement, i.e., during the Rest period and the preceding trials. In addition, and in contrast to cDBS, aDBS had dispersion in inter-stimulus intervals and amplitude. This finding on the train stimulation from aDBS is consistent with the nonstationary behavior of M1-γ2 activity during and between task periods. The dispersion in inter-stimulus intervals and amplitude during aDBS underlies irregular patterns in the stimulus pulse trains. Irregular train stimulations are known to provide clinical benefits comparable to those from cDBS but for different electrical charge 54,55 in experimental and clinical applications 54,[56][57][58] . In these regards, the present findings on M1-γ2 aDBS share similarities with the previous findings on irregular pulse train DBS. Therefore, dispersion features of stimulation pulses in aDBS may be another mechanism underlying the clinical benefits from M1-γ2 aDBS. Importantly, M1-γ2 aDBS showed lower electrical charge delivered during trials in comparison to cDBS. This effect was dependent on the task period, with the lowest relative charge delivered during the Rest period. An important clinical benefit from M1-γ2 aDBS may be to extend the battery lifetime by reducing electricity consumption when the patient is not engaged in motoric activity.
In line with studies focusing on other biomarkers 55 , the following two fundamental questions arise: (1) Is one biomarker better than the others to drive aDBS? (2) Is there a common neurophysiological mechanism  www.nature.com/scientificreports/ underlying the clinical benefits emerging from these different adaptive systems? From our discussion and to answer these questions one will need to assess what are the features of the adaptive stimulation train that results in clinical benefits. A pivotal question is whether the correlation in time between the dynamic of the stimulation and the dynamic of the biomarker is necessary to the clinical benefits from aDBS. This should not be taken for granted since aDBS paradigms share similarity with other paradigms of stimulation such as the variable frequency paradigms 59 . The variable frequency paradigm is an alternative promising approach to aDBS with the presumption of DBS interacting with many intrinsic oscillators 60 to reach broader circuitry effects and clinical benefits. Evidence for this last hypothesis is the dependency of PD symptoms on the frequency of STN-DBS 61 .
For example, bradykinesia responds to stimulation over multiple different frequencies in a nonlinear manner 62 . The benefit of aDBS, comparatively to cDBS, may also result from the larger subset of frequencies multiplexed in the DBS pulse train. The point here is that there is uncertainty regarding the mechanisms underlying the clinical benefits from M1-γ2 aDBS and that future investigations will have to tackle the question of whether the correlation in time between stimulation's dynamic and biomarker's dynamic are necessary to explain the clinical benefits of aDBS. This is certainly an important future direction rendered possible with our brain-machine interface. A possible approach would consist of using surrogate aDBS pulse trains for comparison to the original train stimulation. These surrogate aDBS pulse trains would share similar statistical characteristics to the original aDBS pulse trains, but they would be uncorrelated to the dynamic of the biomarker in time. Such surrogate DBS pulse trains can be generated by shuffling the order of the inter-pulse intervals of the original aDBS pulse train. Comparing the clinical benefits between by the original aDBS stimulation train and by its surrogate may provide valuable information on the mechanism by which aDBS alleviates motor symptoms in PD.
The present brain-machine interface is a tool for investigating aDBS driven by the cortical oscillatory activities in a given frequency band and its application is not limited to the γ2 band. By adjusting the filter setting, this system may be helpful in comparing the clinical benefits between different clinical markers used to drive aDBS. Our aDBS system required few setup parameters and it may be integrated easily into clinical settings such as in the operative room or possibly in clinic 63 . Although chronic intracortical recording is not commonly in use, technical difficulties have been overcome 64 , thus allowing such methodologies to move from clinical research to clinical application 65,66 . In this last case, and in PD patients, special attention should be paid to tremor episodes that are associated with increased γ band activity in the STN 19 and possibly in M1 as well 67 . Since MPTP-treated macaque monkeys do not develop tremor, it remains to be established whether tremor-related activity may also benefit from M1-γ2 aDBS modulation. Also, possible interference may occur from dyskinesia, which may be associated with decreased γ1 activity 3,68 . Therefore, the divergence in cortical oscillations between voluntary movements and involuntary movements in the γ2 band should be evaluated as a factor that may predict clinical benefits in patients with PD and L-dopa-induced dyskinesia.
In summary, M1-γ2 aDBS is a safe strategy to positively modulate the stimulation train when therapeutic needs are presumed to be increased for facilitating voluntary movement. Our brain-machine interface allowed the delivery of higher pulse amplitude and higher pulse rate during the periods of preparation and execution of movement in comparison to the period of rest. M1-γ2 aDBS and cDBS had equivalent clinical benefits and M1-γ2 aDBS delivered only 2/3 of the charge of cDBS. Our study reports the feasibility and clinical benefits of M1-γ2 aDBS modulation in two parkinsonian monkeys. While our data are supportive for M1-γ2 aDBS modulation to require lower charge delivered than cDBS, the energy efficiency of M1-γ2 aDBS remains to be established in regard to the potential energy costs associated with running sensing and signal-to-stimulation processing in PD patients.